Nutrient and Mercury Concentrations and Loads in Lake Tahoe Basin Snowpack
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Approximately seventy percent of Lake Tahoe Basin precipitation falls as snow during the winter and spring. During snowpack storage, chemicals that accumulate throughout the season via wet and dry deposition are subject to transformations and emissions that affect the end-of-season chemical load in runoff and infiltrating groundwater. This study quantified the dynamics of nitrogen (N), phosphorus (P), and mercury (Hg) concentrations and loads in Lake Tahoe Basin snowpack to fill a gap in the watershed's nutrient and pollutant mass balance.Bi-weekly integrated snowpack cores and event-based surface samples were collected at seven sites along two elevation gradients in the Tahoe Basin during the 2011-12 and 2012-13 snow years from the first substantial snowfall until the onset of melt. Snowpack N content was controlled largely by deposition of nitrate (NO<sub>3</sub><super>-</super> ), total ammonia nitrogen (TAN: NH<sub>3</sub> + NH<sub>4</sub><super>+</super>), and organic N. Specifically, organic N comprised 48 percent of all snowpack N, while NO<sub>3</sub><super>-</super> and TAN made up 25 and 27 percent, respectively. NO<sub>3</sub><super>-</super> deposition was positively correlated with snow accumulation and snowpack concentrations were relatively constant throughout the sampling seasons. NO<sub>3</sub><super>-</super> snowpack concentrations had no discernible spatial patterns and were likely dominated by NO<sub>x</sub> emissions from out-of-basin sources and consecutive wet deposition during snowfall. Unlike NO<sub>3</sub><super>-</super> , TAN deposition was associated with both wet and dry deposition as evident by increasing snowpack concentrations during snowfall-free periods and concentrations increased towards the end of winter. The late-season increase of TAN was likely due to increased vertical mixing of the boundary layer and the onset of agricultural activity in the San Joaquin Valley. Organic N concentrations in snowpack were highly variable and showed no clear temporal or spatial dependence throughout the sampling season. P deposition was strongly correlated with location in the basin (i.e. east/west) and elevation. Specifically, P concentrations were higher on the east side of the basin than the west side and decreased with increasing elevation. The spatial patterns of P are consistent with particulate-bound dry deposition originating mainly from in-basin urban sources, which also was reflected by urban areas showing by far the highest snowpack P concentrations. Lastly, Hg deposition showed little spatial or temporal variability throughout the basin. Hg speciation showed a post-depositional shift from dissolved to particulate-phase as the dominant form. This shift was likely driven by photo-chemically induced gaseous re-emission of dissolved Hg and dry deposition of particulate Hg during storm free periods. Along with this change, snowpack Hg concentrations increased with elevation, possibly due a combination of increased wet deposition and limited light penetration into deeper snowpack causing reduced photochemical re-emission. Therefore, snowpack Hg chemistry showed complex patterns in Hg not only related to total precipitation, but further affected by photochemical reduction and changes in Hg speciation over time.Basin-wide mass loading estimates show substantial accumulation of pollutants and nutrients within Lake Tahoe snowpack each year. Based on average annual maximum snow accumulation from 2000 to 2011, peak snowpack total N loading ranged from approximately 200 to 4127 g ha<super>-1</super> and peak snowpack P loading ranged from 16 to 628 g ha<super>-1</super>. In comparison with annual lake pollutant loading estimates, snowpack total N and P storage represents approximately 63 and 24 percent of previous estimates of land-based inputs to the lake, respectively. Peak snowpack total Hg loads ranged from 2.6 to 136 mg ha<super>-1</super>, similar to other loading estimates in the Sierra Nevada Mountains just outside the basin. Study results reflect the highly dynamic nature of snowpack chemical storage and identify snowpack chemical loading from atmospheric deposition as a substantial source of nutrients and pollutants within the Lake Tahoe watershed each year.